Laser containing a distributed gain medium

ABSTRACT

A laser device which may be used as an oscillator or amplifier comprising a chamber having a volume formed therein and a gain medium within the volume. The gain medium comprises a solid-state element containing active laser ion within the volume. A cooling fluid flows about the solid-state element and a semiconductor laser diode provides optical pump radiation into the volume of the laser chamber such that laser emission from the device passes through the gain medium and the fluid. The laser device provides the advantages of a solid-state gain medium laser (e.g., diode-pumping, high power density, etc), but enables operation at higher average power and beam quality than would be achievable from a pure solid-state medium.

This application is a continuation of U.S. application Ser. No.11/190,357, filed Jul. 26, 2005, entitled, LASER CONTAINING ADISTRIBUTED GAIN MEDIUM, now U.S. Pat. No. 7,103,078 which is acontinuation of U.S. application Ser. No. 10/302,630, filed Nov. 21,2002, entitled, LASER CONTAINING A DISTRIBUTED GAIN MEDIUM, now U.S.Pat. No. 6,937,629, which claims the benefit of the followingapplications: U.S. Provisional Application No. 60/332,085, filed Nov.21, 2001, entitled LASER CONTAINING A SLURRY; and U.S. ProvisionalApplication No. 60/401,411, filed Aug. 6, 2002, entitled LASERCONTAINING A DISTRIBUTED GAIN MEDIUM, the entire disclosures of allapplications incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of laser systems, and morespecifically, it relates to laser systems in which the gain medium isdiode-pumped.

2. Background

Lasers have been produced from a variety of materials and in all phases:liquid, gas, plasma and solid-state. Liquid lasers most commonly containof a gain medium of organic dye salts such as derivatives of Rhodamineor fluorescein which are dissolved in solvents such as methanol orwater. The gain medium is excited by light produced typically byflashlamps or another laser. These dye lasers exhibit gain across abroad region of the visible to near infrared spectrum. The high gaincross section and broad tunability make these lasers attractive for anumber of uses. On the other hand, these dye lasers exhibit a very shortupper-state lifetime and low saturation fluence which makes themunsuitable for high energy applications. Furthermore, the absorptionbands of these dyes do not overlap with the emission bands of high powersemiconductor diode lasers (typically 780 to 980 nm) and are thereforenot amenable to pumping with currently available laser diodes.

There have been various attempts at achieving inorganic liquid laserscomposed of rare earth salts dissolved in various solvents. Thechallenge in these systems has been the tendency of the lasing ion(e.g., Nd³⁺) to undergo radiationless decay from the upper laser levelin most liquid solutions. This de-excitation is due to the high energyvibrations of bonds involving light atoms such as hydrogen. Byeliminating the presence of any light atoms, this quenching can bereduced to the point where lasing can be achieved. For example, thisquenching has been used to achieve lasing in Neodymium SeleniumOxychloride.

A significant challenge with such liquid lasers is the corrosive natureof the solvents which requires special handling both the preparation anduse of the laser medium. When initially developed, these systems wereinitially pumped by flashlamps. By using recently developed laser diodearrays as pump sources, the pump light can match the absorption spectrumthereby enabling a resurgence of interest in this type of liquid laser.

Solid-state lasers contain a gain medium which is comprised of a lasingion contained in a crystal or amorphous matrix. The most common lasingspecies are based on rare-earth elements such as neodymium, erbium,ytterbium, etc. The laser properties (e.g., absorption and emissioncross-section, upper-state lifetime, etc.) of the gain medium aredetermined by the interaction of the local crystal field with the fieldof the ion itself. This interaction determines the specific energylevels of the ion and their width. For example, the neodymium ionexhibits a series of narrow absorption lines centered around 808 nm anda series of strong emission lines around 1064 nm when bound in thecommon crystal matrix, yttrium aluminum garnet (YAG). This laser mediumis formed when a small amount of neodymium replaces the yttrium ion inthe garnet crystal matrix resulting in neodymium-doped YAG (Nd:YAG). Byplacing the neodymium in other crystal hosts, very different laserproperties can be obtained. For example, neodymium dissolved inphosphate glass will produce a series of broad absorption bandsextending from 500 to over 900 nm and tens of nanometers in width. Gainis exhibited across a broad band centered at 1054 nm. Unlike dye lasers,these solid-state lasers exhibit a long upper-state lifetime and a highsaturation fluence which enables high energy output. In addition, theirabsorption bands in the 800 to over 950 nm range make them ideal forexcitation by the emission from semiconductor laser diodes based ongallium arsenide (e.g., AlGaAs, InGaAs).

Solid-state lasers are formed by growing the crystal (or melting theglass) and then cutting and polishing the crystal into the desiredshape. Large scale, single-pulse systems with the solid-state media inthe shape of disks have been developed for laser-driven inertialconfinement fusion research with disks over 40 cm in diameter. Thesesystems produce very high pulse energy (approximately 15 kJ) butdisadvantageously, have low average power (less than 1 W). High averagepower systems containing either Nd:YAG or Yb:YAG as the gain medium andpumped by laser diodes have been developed at the kilowatt level. Atypical diode-pumped solid-state laser is shown in FIG. 2. The solidstate laser 200 includes a laser rod 202 surrounded by a flow tube 204(sheath) which directs a coolant fluid flow 206 across the laser rod 202and prevents contact of the laser rod 202 with diode pump sources 208.Pump radiation is produced by the diode arrays 208 which may be coupledinto the laser rod 202 by a refracting means 210 (e.g., lenses) or otherreflecting means. The coolant fluid removes heat from the laser rod 202through convection by flowing the fluid 206 over the surface of thelaser rod 202. Heat is conducted through the laser rod to the surface.This conduction establishes a large temperature gradient between thecenter of the rod and the surface. This gradient causes stress withinthe material which results in beam distortion and eventuallycatastrophic failure of the crystal. As a result, the average poweravailable from solid-state lasers is limited by the ability to removeheat from the medium.

A class of solid-state lasers which contained the solid materialimmersed in a fluid were developed in the 1970's and 1980's. These socalled “immersion lasers” were flashlamp-pumped and had the solid-statelaser material shaped into various forms. For example, as described inU.S. Pat. No. 3,735,282 issued to Gans, a pulsed laser is describedwhich is composed of a segmented Nd:Glass rod immersed in a liquid whichis index matched at the emission wavelength. The liquid consists ofbrominated acyclic hydrocarbon mixed with acyclic alcohol. The segmentedrod is immersed in a thermostatically controlled housing surrounded by ahelical arc lamp containing xenon or krypton. Gans focuses on aparticular type of index matching fluid containing OH and bromine groupsto prevent ultraviolet hydrolysis of the index matching fluid.

A similar laser architecture can be found in U.S. Pat. No. 3,602,836issued to Young, which teaches the use of a segmented laser rod immersedin a coolant fluid. Young focuses on meniscus-shaped segments of zerolens power spaced apart a sufficient distance to permit free passage ofsufficient coolant. In U.S. Pat. No. 3,621,456 issued to Young, the useof parallel discs containing reflective surfaces is described. In U.S.Pat. No. 3,487,330, issued to Gudmundsen, a laser arrangement isdescribed in which the flashlamp is enclosed by segmented lasermaterial. In such a system, the coolant flow would be directed inwardlyacross the segmented laser materials so that the unheated coolant firstcrosses the laser material and then cools the lamp. Additionally,Gudmundsen also describes that the flashlamp is placed adjacent to thesegment laser material. Gudmundsen primarily focuses on various coolantflow geometries for cooling the laser media and the flashlamp.

A packed bed laser composed of solid state glass lasing elements isdescribed in U.S. Pat. No. 4,803,439 issued to Ryan, et al. Ryan et al.teach the use of lasing beads which are packed within a laser cavity tobe in contiguous contact with each other. The laser beads are to be ofthe order of 1 cubic millimeter in volume to facilitate the packing ofthe glass lasing beads contiguously into the laser amplifier cavity. Acooling fluid is pumped through and in between the lasing beads whilethe beads are fixed in space. Also, a phase conjugate mirror is requiredto cancel the optical distortions associated with the lasing medium.

All of these solid-state laser systems including the so-called immersionlasers are limited in average power output by heat removal from the gainmedia. The difference between the energy of the pump photon and theemission photon is referred to as the quantum defect and left in thecrystal as heat. For example, Nd:YAG is pumped by laser diode radiationat 808 nm (photon energy=1.53 eV) and emits laser radiation at 1064 nm(1.17 eV), the quantum defect of 0.36 eV appears as heat within themedium following lasing. This heat must be removed or will terminatelasing by thermally populating the lower laser level (e.g., Yb:YAG) oreventually resulting in catastrophic failure of the laser crystal bythermal stresses associated with the temperature gradient across thecrystal. Severe beam distortion and depolarization resulting from thetemperature dependence of the refractive index and stress birefringenceoccur far below the limit of thermal stress induced fracture. Heat iscommonly removed by flowing a coolant across the laser material.Alternate heat removal methods designed to address the problem ofthermal stress and beam distortion have led to a variety of laserdesigns, such as thin-disk and zig-zag slab solid state lasers.

What is needed is a laser device in which the advantages of asolid-state gain medium (e.g., diode-pumping, high power density, etc)can be realized but which is not limited in average power output bythermal stress.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as wellas other needs by providing a laser device which addresses the thermallimitations in solid state lasers and provides high average power. Sucha laser is generally not limited by thermal stress or other thermaleffects in the solid-state gain medium.

In one embodiment, the invention may be characterized as a laser devicelaser device comprising a laser chamber having a volume formed thereinand a gain medium within the volume and comprising a solid-state elementcontaining active laser ion within the volume, the solid-state elementrigidly held in a fixed position within the volume. The laser alsocomprises a fluid flowing about the solid-state element; a semiconductorlaser diode for providing optical pump radiation into the volume of thelaser chamber; and a laser emission from the solid state element passesthrough the gain medium and the fluid. A dimension L of the solid stateelement is defined by:

$L \leq \sqrt{\frac{\Delta\;{Tck}_{s}}{q^{''\prime}}}$where ΔT is a maximum temperature difference between a center and asurface of the solid state element, k_(s) is a thermal conductivity ofthe solid state element, c is a geometric constant wherein c is a valuebetween 4 and 8, and q′″ is a heat generated per unit volume within thesolid state element resulting from diode-pumping.

In another embodiment, the invention may be characterized as a method oflasing comprising the steps of: providing a laser chamber having avolume formed therein and containing a gain medium, the gain mediumcomprising a solid state element containing active laser ion within thevolume, the solid state element rigidly held in a fixed position withinthe volume; flowing a cooling fluid through the volume and about thesolid state element; directing optical pump radiation produced bysemiconductor laser diodes through the chamber into the volume; anddirecting a laser emission produced by the solid state element throughthe chamber such that the laser emission passes through the solid stateelement and the fluid. The providing step comprises providing the laserchamber containing the gain medium, wherein a dimension L of the solidstate element is defined by:

$L \leq \sqrt{\frac{\Delta\;{Tck}_{s}}{q^{''\prime}}}$where ΔT is a maximum temperature difference between a center and asurface of the solid state element k_(s) is a thermal conductivity ofthe solid state element, c is a geometric constant wherein c is a valuebetween 4 and 8, and q′″ is a heat generated per unit volume within thesolid state element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system view of a laser device in accordance with oneembodiment of the present invention in which a gain medium comprisesrare-earth doped solid-state elements, embodied as flat sheets, fixed inposition within a laser chamber and distributed throughout a coolingfluid which exhibits a refractive index similar to that of thesolid-state gain material. Flow of the cooling fluid through the laserchamber is across the solid-state elements.

FIG. 2 is a diode-pumped, solid-state laser gain assembly representativeof the current art, including a laser rod surrounded by a flow tubewhich direct coolant flow across the laser rod and prevents contact witha diode pump source. Pump radiation is produced by the diode arrays andmay be coupled into the laser rod by refracting (e.g., lenses) orreflecting means.

FIG. 3 is an external side system view of an alternative laser device inaccordance with another embodiment of the invention in which the coolantfluid is flowed through the laser chamber by a manifold assembly.

FIG. 4 is an enlarged cross sectional view of the laser chamber of thelaser device of FIG. 3 further illustrating the solid-state elements andsemiconductor diode pump sources.

FIG. 5 is a side cross sectional view of the laser device of FIG. 4taken along line 5-5 of FIG. 4 further illustrating a fluid flowmanifold for flowing the fluid through the laser chamber.

FIG. 6 is a diagram illustrating the orientation of solid state elementsembodied as sheets in accordance with an embodiment of the invention.

FIG. 7 illustrates an example of one embodiment of a laser device inaccordance with one embodiment of the current invention used as afour-pass amplifier.

DETAILED DESCRIPTION OF THE INVENTION

It is an objective of several embodiments of the present invention toprovide a diode-pumped, laser in which the average power of the laser ismuch greater than can be achieved by current solid-state systems whichare limited by thermal stress or other thermal effects in thesolid-state gain medium.

It is an objective of several embodiments of the present invention toprovide a diode-pumped laser which may be operated continuously orpulsed.

It is also an objective of several embodiments of the present inventionto provide a laser in which the solid-state gain medium is distributedthroughout a flowing coolant and in intimate contact with it.

A laser in accordance with several embodiments of the invention can beused either as the gain medium for a laser oscillator or as anamplifier.

As described previously, there are a multitude of geometries fordiode-pumped solid-state lasers. These configurations all utilize asolid-state gain medium which has been configured as a rod, slab, ordisk and heat is removed from the gain medium either by conduction to athermal reservoir or by convection to a coolant flowing over thesurface. To our knowledge, there are no configurations of high power,diode-pumped solid-state lasers in which the laser emission axis passesthrough the coolant. High power operation is considered herein to referto laser average power in excess of 100 W. In conventionalconfigurations, temperature gradients cause stress within the gainmedium resulting in beam distortion and eventually catastrophic failureof the laser if the average power is not limited. It is an objective ofseveral embodiments of the present invention to provide a diode-pumpedlaser architecture with a large gain aperture which enables both side orend-pumping architectures and achieves high extraction efficiency andhigher average power operation than can be achieved from currentstate-of the art side-pumped or end-pumped laser rods or slabs.

According to one embodiment, a solid-state laser gain medium comprises aplurality of solid state elements fixed in position within a laserchamber having a volume formed therein. The plurality of solid stateelements are immersed in a liquid coolant or fluid which is flowed aboutthe solid state elements. In some embodiments, the fluid issubstantially index-matched to the solid state elements, while in otherembodiments, the fluid is not required to be substantially indexmatched. The electromagnetic wave or laser emission passes through thesolid state elements and the fluid. This is in contrast to conventionalhigh power solid state lasers, such as illustrated in FIG. 2, where thecoolant fluid flows about the solid state gain medium while the laseremission does not pass through the coolant. According to severalembodiments of the invention, the gain medium and coolant fluid arehomogeneous to the electromagnetic wave and enables the use of asolid-state laser gain medium in which each of the solid state elementshave a dimension sufficiently small that heat is rapidly conducted outof the solid-state element into the liquid coolant. Thus, such a laserdevice provides the advantages of a solid-state gain medium laser (e.g.,diode-pumping, high power density, etc), but enables operation at higheraverage power and beam quality than would be achievable from a puresolid-state medium.

Referring to FIG. 1, one embodiment of a laser device 10 is illustrated.The laser device 10 includes a laser chamber 12 (also referred to as alaser head) having a volume (also referred to as a lasing region) formedtherein. A gain medium comprising a plurality of solid-state elements 14is distributed within the volume. For example, the elements 14 areembodied as flat sheets or plates of solid state material that aredistributed or spaced within the volume of the laser chamber 12.According to one embodiment, the elements 14 of solid-state gain mediumsuch as rare-earth (e.g., neodymium) doped glass, yttrium lithiumfluoride (Nd:YLF) or other laser host, are distributed inside the volumeand a coolant fluid 16 is flowed about the elements 14. The fluid 16serves as the coolant for the laser medium or elements 14. The elements14 are thin in one dimension in order to readily conduct the heat out ofthe solid-state medium into the coolant fluid 16. In some embodiments,the fluid has a refractive index which is approximately index matched tothat of the solid state elements 14. As used herein, a fluid isapproximately or substantially index matched to the refractive index ofthe gain medium material when the refractive index is withinapproximately 20% of the refractive index of the solid state materialselected. It is noted that although the elements are primarily describedherein as flat sheets or plates, the elements may take on othergeometries (e.g., curved, elongated, rounded, etc.) in accordance withseveral embodiments of the invention.

The fluid 16 generally flows into the laser chamber 12 via an inlet 20(illustrated on a top surface of the laser chamber 12) and exits thelaser chamber 12 via an outlet (not shown in FIG. 1). In thisembodiment, the fluid flows vertically from top to bottom across thelaser emission axis through the volume (e.g., along the z-axis of FIG.1). The elements 14 are rigidly held within the volume in a structure(not shown) and the coolant fluid passes in between the elements. Forexample, the elements 14 are held in position by sleeves or otherstructures coupled to interior portions of the chamber 12, or are heldwithin grooves formed, for example, in an interior portion of thechamber walls. The elements 14 are preferably sized such that they fillthe laser aperture in the large dimension and are sufficiently thin inthe small dimension that the temperature gradients within the elementsremain far below that imposed by any thermal stress limit in thematerial. Although discussed in more detail below, the small dimensionis preferably less than approximately 1 centimeter and typically in therange of a few microns to a few millimeters, e.g., between 0.01 mm to 10mm. The elements 14 are excited by pump radiation from arrays ofsemiconductor laser diodes 18. In the embodiment shown in FIG. 1, diodepump radiation is introduced into the laser chamber 12 through its sides(e.g., along the y-axis).

According to one embodiment, the laser chamber 12 has windows 24 orwalls composed of transparent optical material such as fused silica,borosilicate glass or sapphire and also has opaque portions 26 or wallsections. However, it is noted that the chamber 12 may be made entirelyof transparent optical material. The chamber 12 may take on a variety ofgeometries, such as a rectilinear parallelepiped as illustrated, or acylindrical geometry. For example, the laser chamber 12 includes wallshaving a thickness and forming the laser chamber with the volumetherein. Diode produced pump radiation from the semiconductor laserdiodes 18 passes through the windows 24 formed in the side walls of thelaser chamber 12. The ends of the laser chamber 12 are composed ofoptical material transparent to the laser wavelength and form aperturewindows 28 for the laser emission to enter and exit the laser chamber 12along a laser emission axis. Depending on the embodiment, the inventioncan be used either as a laser oscillator or as an amplifier. The chambercontaining the gain medium is placed within a cavity composed of a highreflector 30 and an output coupler 32 (also referred to as a partiallytransmitting mirror) when the invention is configured as a laseroscillator.

Another embodiment of the present invention is shown in the externalside system view of FIG. 3, which is similar to the embodiment of FIG.1; however, the coolant fluid 16 is flowed through the laser chamber 12using a manifold assembly. For example, an inlet manifold 38 located ontop of the laser chamber is coupled to several inlets 20 to the laserchamber 12, while an outlet manifold 40 located on the bottom of thechamber 12 is coupled to several outlets 22 of the laser chamber 12. Itis noted that the inlet manifold 38 and the outlet manifold 40 as wellas the inlets 20 and the outlets 22 are better illustrated in FIG.5,which will be described in more detail below. The purpose of themanifold is to distribute the fluid uniformly throughout the laserchamber or head and maintain a uniform flow velocity and pressure. It isnoted that FIGS. 4 and 5 better illustrate the laser chamber 12 of theembodiment of FIG. 3.

Referring to both FIGS. 1 and 3, the coolant fluid 16 is pumped into thevolume of the laser chamber 12 through a pipe 34 coupled to the one ormore inlets 20 of the chamber 12. For example, in FIG. 1, the pipe 34 isdirectly coupled to the inlet 20, while in FIG. 3, the pipe is coupledto the inlet manifold 38 which is coupled to multiple inlets 20. A fluidpump 36 is a common centrifugal pump with seals designed forcompatibility with the index matching fluid 16. The fluid 16 flows fromthe pump 36 into the laser chamber 12 via the pipe 34. In thisembodiment, the piping 34/inlet manifold 38 are configured to introducethe coolant 16 into the laser chamber 12 at a shallow angle relative tothe elements 14 so as not to create large scale turbulent eddies againstthe solid state elements 14. The minimum flow velocity set by the pump36 is that required to achieve adequate cooling of the elements 14. Asbest illustrated in FIG. 5, the coolant fluid 16 flows across the laseremission axis (i.e., the laser emission passes through the fluid 16) andabout the elements 14 (e.g., in between adjacent elements 14) and flowsout of the laser chamber 12 through one or more outlets 22 coupleddirectly to a pipe 42 or coupled to an outlet manifold 40 located on theopposite side of the laser chamber 12, which is coupled to the pipe 42.The direction of fluid flow is illustrated by arrows F in FIGS. 1, 3 and5. Within the laser chamber 12, the elements 14 are excited by opticalpump radiation from the arrays of diodes 18. As a result of thisexcitation, the temperature within the elements increases. This heat istransferred to the flowing coolant fluid 16 by convection at the surfaceof the individual elements 14. The coolant fluid exiting the laserchamber 12 is directed to a heat exchanger 44 via the piping 42 coupledthere between. The heat exchanger 44 cools the fluid 16 back to itsstarting temperature and then passes the fluid back to the pump 36. Theheat exchanger 44 is most commonly a tube type exchanger where the hotfluid is cooled by conduction through one or a series of tubes, whichare, in turn, cooled by a secondary fluid. The secondary cooling fluidis most commonly water or air. Following the heat exchanger, the coolantflows back to the pump 36 completing the flow cycle. It is noted thatdepending on the embodiment, the fluid may flow through the either ofthe pump 36 or the heat exchanger 44 prior to flowing the other.

It is noted that while referring to the embodiments of FIGS. 1 and 3,concurrent reference will be made to FIGS. 4 and 5. FIG. 4 is anenlarged cross sectional view of the laser chamber of the laser deviceof FIG. 3 (or of FIG. 1) further illustrating the orientation of thesolid-state elements 14 and semiconductor diode pump sources 18. FIG. 5is a side cross sectional view of the laser device of FIG. 3 taken alongline 5-5 of FIG. 4 further illustrating a fluid flow manifold system forflowing the fluid through the laser chamber.

As can be seen in FIG. 4, in another embodiment, the elements 14 areoriented at an angle relative to the path of the laser emission (thelaser emission illustrated by arrows 46 in FIGS. 4 and 5), preferably atBrewster's angle. At this orientation, the fluid is not required to besubstantially index-matched to that of the solid state elements 14.However, it is noted that such orientation is not required, and theelements 14 may be oriented at any angle (including normal) to the pathof the laser emission. FIG. 5 clearly illustrates the top to bottomfluid flow path (arrows F) within the laser chamber 12 and about thesolid state elements 14. Preferably, the fluid flows straight throughthe chamber from the inlets 20 to the outlets 22. It is noted thatgenerally, the elements 14 are oriented parallel to the direction of thefluid flow such that the fluid flows about the entire surface area ofthe length and width of the elements 14 across the volume. It is furthernoted that since the view of FIG. 5 is a cross section taken along line5-5 of FIG. 4, the angular orientation of the elements 14 relative tothe laser emission path is not illustrated. However, the elements 14actually extend diagonally in the y-axis of FIG. 5. Again, in contrastto known solid state lasers as illustrated in FIG. 2, the laser emissionfrom the laser device 10 is intended to pass directly through thecoolant fluid 16.

An enlargement of the laser chamber 12 is shown in FIGS. 4 and 5 Thelaser cavity is formed on one end by a high reflector 30 which is amultilayer dielectric coated mirror designed to be a high reflector atthe laser wavelength. In alternate embodiments, this mirror can becomprised of a metallic or semiconductor reflector which may or may notcontain a dielectric coating. The gain medium is comprised of thesolid-state elements 14 (e.g., plates or sheets) in the cooling fluid 16which is bounded on both ends of the cavity by the optical window 28(also referred to as an aperture). In one embodiment, Fresnel losses offof the windows 28 may be minimized by orienting the windows 28 atBrewster's angle relative to the laser emission. At this angle, lightwhich is polarized in the plane of the window 28 passes through with noreflection at the window's surface even when the surface is uncoated. Inthe preferred embodiment, the window 28 is made of laser grade qualityand is made from either fused silica, sapphire or borosilicate glass. Atthe other end of the cavity is the output coupler 32. The output coupler32 is a partially reflecting mirror which transmits some of the incidentlaser radiation. In the preferred embodiment, this mirror is comprisedof a multilayer dielectric coating deposited on a fused silica substrateand designed to transmit 10 to 80% of the incident laser radiation. Theoptimum transmission and geometry of this output coupler 32 will bedetermined by the single-pass gain within the cavity which is a functionof the available pump power from the diodes 18, the concentration oflasing ion (e.g., neodymium) and the length of the gain region andoverall cavity design requirements which are well known in the art. Thegain medium is excited by pump radiation (illustrated as arrows P inFIG. 4) from arrays of diodes 18 arranged on the side of the gainregion. The diode arrays are designed to emit in the absorption band ofthe solid-state elements 14 (e.g., near 800 nm for neodymium). The pumpradiation from the diode arrays passes through a window 24 in the sideof the gain region which is formed from either fused silica,borosilicate glass or other optical materials known in the art.

As described above, the gain medium (i.e., the solid state elements 14)and fluid 16 appear nearly homogeneous to the electromagnetic wave sincethe coolant 16 is in intimate contact with the solid-state elements andin some embodiments, is approximately index matched to it. Laseroscillation occurs between the high reflector 30 and the output coupler32. The output coupler 32 is a partially transparent mirror which iscommonly composed of dielectric layers alternating in refractive index.The high reflector 30 is also well known in the art and is typically amultilayer dielectric coated mirror. Metallic mirrors can also be used.In addition, a deformable mirror can be used in the location of the highreflector. When coupled with a wavefront sensor, such a mirror can beused to compensate for any distortion in the laser head resulting fromflow of the fluid or temperature gradients.

As illustrated in FIG. 5, the coolant is flowed transverse to thedirection of the laser axis of the laser emission (the emissionillustrated as arrows 46 in FIGS. 4 and 5). Radiation from laser diode18 arrays at a wavelength tuned to the absorption band of thesolid-state medium enters through a window 24 on the side of the laserchamber containing the elements 14. In some embodiments, anapproximately index-matched coolant is selected such that it exhibitsnegligible absorption at both the absorption and emission wavelength ofthe solid-state gain medium (e.g., for Nd:glass λ_(a)≈803 nm andλ_(em)≈1054 nm). The radiation from the pump diodes 18 is absorbedwithin the solid-state elements 14 which are held in position within thelaser chamber. In the illustrated embodiment, the elements 14 areoriented parallel to each; however, it is not necessary that theelements 14 be oriented parallel to each other. The only requirement isthat they are sufficiently separated or oriented from one another thatthe coolant fluid 16 may pass readily between adjacent elements 14.

In addition to exhibiting negligible absorption, in one embodiment, thefluid 16 is approximately index matched to that of the solid-statemedium, i.e., the elements 14, in order that the losses resulting fromthe difference in refractive index between two different media aresufficiently small to not significantly effect the efficiency of thelaser or the net single-pass gain. These losses are due to reflection atthe interface and are known in the art as Fresnel losses. There areindex matching fluids 16 available within the laser community. Whilecommercially available index matching fluids may be used, simple mixingof miscible fluids: one with an index above that of the gain medium andone below can be used. In one embodiment, the fluids are mixed in theratio determined byn ₁ x+n ₂(1−x)=n _(p)where

-   -   n₁=refractive index of fluid 1    -   n₂=refractive index of fluid 2    -   n_(p)=refractive index of the solid element    -   x=mixing fraction

An example would be the mixture of carbon tetrachloride and carbondisulfide to provide an index matched fluid for neodymium-dopedphosphate glass elements. Carbon disulfide has a refractive index at1054 nm of 1.62 and carbon tetrachloride has an index of 1.45 while theNd-doped phosphate glass LHG-5 has a refractive index of 1.531. Bymixing carbon disulfide and carbon tetrachloride with 47.65% CS₂, anindex matched coolant can be obtained. Thus, according to preferredembodiments, carbon-based coolant fluids 16 are used.

For some materials, pure fluids can exhibit a refractive indexsufficiently close to that of the solid elements that they may be useddirectly. A specific example would be the use of carbon tetrachloride(refractive index=1.45) with Nd:YLF (index along ordinary axis=1.4481).The reflection loss (Fresnel loss) at the interface between the coolantfluid 16 and the solid medium (e.g., elements 14) for light which ispolarized perpendicular to the plane of incidence is given by,

$R_{\bot} = \left\lbrack \frac{{n_{i}\cos\;\theta_{i}} - {n_{t}\cos\;\theta_{t}}}{{n_{i}\cos\;\theta_{i}} + {n_{t}\cos\;\theta_{t}}} \right\rbrack^{2}$and, for light which is polarized parallel to the plane of incidence,

$R_{\parallel} = \left\lbrack \frac{{n_{i}\cos\;\theta_{t}} - {n_{t}\cos\;\theta_{i}}}{{n_{i}\cos\;\theta_{t}} + {n_{t}\cos\;\theta_{i}}} \right\rbrack^{2}$where n_(i) is the index of refraction for the medium in which the lightis incident, n_(t) is the index of refraction of the medium in which thelight is transmitted, θ_(i) is the angle of incidence and θ_(t) is theangle between the normal to the surface and the propagation vector ofthe light in the transmitted medium. These angles are related by Snell'slaw, n_(i) sinθ_(i)=n_(t) sinθ_(t). For light which is incident normalto the sheet, θ_(i)=θ_(t)=0, have a loss of much less than one part in10,000 at the surface for the case of Nd:YLF immersed in carbontetrachloride. Even if 100 individual elements 14 were implementedwithin the laser chamber 12, the total Fresnel loss would be less than afew percent for example.

It is noted that a much greater difference can be tolerated in therefractive index between the coolant fluid 16 and the solid state gainelements 14 by simply orienting the elements 14 at an angle with respectto the laser emission axis. The orientation of the elements 14 relativeto the laser emission axis is best illustrated in FIGS. 1 and 4. At aspecific angle, known as Brewster's angle, the reflection of lightpolarized parallel to the plane of incidence goes to zero. This angle,θ_(B) is given by θ_(B)=arc tan (n_(t)/n_(i)). Hence, by orienting theelements 14 at Brewster's angle, coolants could be used which exhibit avery large difference in refractive index from the solid-state lasermedium.

To complete the design according to several embodiments, theconcentration of lasing ion in the solid state elements 14, the numberof elements, the aperture size, the element thickness and the spacingbetween elements 14 should be specified. First, in the side-pumpedembodiments shown in FIGS. 1 and 2-5, the concentration of lasing ionshould be sufficiently high in the elements 14 to absorb the majority ofpump radiation from the diodes 18. We will typically adjust the apertureof the gain region (e.g., adjust the size of the windows 28 and internalvolume of the laser chamber 12) such that approximately 90% of the diodelight is absorbed by the elements 14. According to one embodiment, thisrequirement can be written as exp[α_(a) L sin φ]≈0.08, where L is theeffective absorption coefficient for pump radiation. The angle, φ is theangle that the element is oriented to pump radiation from the diodearrays. The effective absorption coefficient, α_(a) is given byα_(a)=N_(o)σwhere

-   -   N_(o)=Concentration of absorbing ions in the element    -   σ_(a)=Absorption cross section        For example, LG-760 Nd:Phosphate laser glass doped at 5 wt % has        a neodymium concentration of 4.65×10²⁰ ions/cm³ and an effective        absorption cross section over the 803 nm band of 2.4×10⁻²⁰ cm²,        leading to an absorption coefficient of 11 cm⁻¹.

The spacing of the elements 14 is adjustable. In preferred embodiments,such as best illustrated in the enlarged diagram of FIG. 6, the elements14 are located such that the elements 14 overlap each other once whenviewed from the perspective of the incident pump radiation P. Thisembodiment allows for the most uniform pumping of the elements 14,allowing for high extraction efficiency in the laser system and lowstress in the material. It is noted that the amount of overlap ofelements 14 may be adjusted depending on the specific implementation.However, if that is the case the overlapped distance should be r/m whenr is the vertical projection of the entire elements and m is an integer.This ensures that the pump light will always see the same absorptionlength all along the gain length. Other spacing between elements 14could be used with associated losses and higher stresses in thematerial. It is noted that although in preferred embodiments, theelements are made to overlap each other from the perspective of theincident pump radiation, in other embodiments, the elements 14 may notoverlap each other at all. Also illustrated in FIG. 5, the elements 14are oriented at an angle θ relative to the laser emission axis(preferably, Brewster's angle) and oriented at angle φ relative to theincident diode pump radiation, the elements having a length 1 and athickness L. As described above, the thickness L should be carefullydesigned in order to allow effective heat removal from the element 14during the lasing action.

The effective small-signal gain coefficient along the laser emissionaxis, α_(e)L, is given by,α_(e) L=Nσ _(e)(N _(sheets L/cos φ))where σ_(e) is the emission cross section, N is the concentration ofions within the solid state element 14 which have been excited by thepump radiation, N_(sheets) is the number of elements 14 within the laserchamber which have been pumped by diode radiation and L/cos φ is theprojection of an element of thickness, L, along the laser emission axis.The ion concentration which is excited is approximately, N=α_(a)φ_(p)(x,y,z) τ_(P) where φ_(p)(x,y,z) is the local photon flux from thepump diodes at the point (x,y,z) within the laser chamber 12 and τ_(p)is the duration of the pump light. This approximation of N=α_(a)φ_(p)(x,y,z) τ_(p) is applicable for pulsed diode pumping and neglectsany spontaneous or stimulated emission during the pump duration. Theexcited ion density is more difficult to calculate in the case ofcontinuous pumping due to the necessity to account for spontaneousemission.

Current diode bars (e.g., semiconductor diodes 18) are able to produceup to 80 W continuous output power from a 1 cm length. For a 2 cm wideby 20 cm long gain region, current state of the art diode pump arrayscould produce a pump power in the range 15-20 kilowatts. TABLES 1 and 2shows the expected gain for neodymium-doped glass and Nd:YLF elements 14spaced to just overlap each other for a total length of the laserchamber of 10 and 20 cm, respectively as a function of continuous diodepumping (per bar output power). The gain is much higher for YLF due toboth a longer upper-state lifetime and a larger stimulated emissioncross section.

TABLE 1 Calculated gain for Nd-doped phosphate glass system Power perBar 60 W 70 W 80 W Average population inversion 1.01e18 1.18e18 1.35e18Single pass gain (10 cm) 1.08 1.10 1.12 4 pass gain (10 cm) 1.36 1.461.57 Single pass gain (20 cm) 1.18 1.21 1.25 4 pass gain (20 cm) 1.942.14 2.44

TABLE 2 Calculated gain for the Nd; YLF system Power per Bar 60 W 70 W80 W Average population inversion 1.38e18 1.61e18 1.81e18 Single passgain (10 cm) 1.65 1.79 1.95 Single pass gain (20 cm) 2.73 3.23 3.82

In the embodiment illustrated in FIGS. 1 and 3-5 and Tables 1 and 2, thelaser chamber 12 was pumped from two sides at 400 W/cm². Since the widthand length of the laser chamber 12 are set by absorption and gain,respectively, the only variable dimension in this embodiment is theheight. In principle, any height could be used when pumping from the twosides. However, diffraction of the beam is minimized when the aspectratio of the aperture (volume) is approximately 1:1. Hence, in preferredembodiments, good laser performance with minimal divergence would beobtained when the height is approximately equal to the width. For ourpresent example, the height would therefore be approximately 2 cm. Thetotal diode power would then be approximately 16 kW from each side.Current state of the art diode arrays contain 1 cm long bars placed on a1.7 mm pitch. With 2 mm between the bars, we would have two blocks ofdiode arrays, each of which is 12 bars high and 17 bars long on eachside of the laser chamber. The total number of bars on each side is then204. To achieve 16 kW pump, will require each bar to operate atapproximately 80 W for the duration of the pump pulse.

TABLE 3 below illustrates several different possible materials for thegain medium and their properties. In addition to the laser materialsshown in TABLE 3, we have also evaluated the performance of erbium andytterbium-doped laser materials for use in several embodiments of thecurrent invention. The design concepts described previously would enableanyone of ordinary skill in the art to produce a laser utilizing thinsolid-state laser elements distributed in a coolant as the gain medium.

TABLE 3 Material characteristics Nd: YLF Nd: YAG σ π Nd: Glass - LG-760Nd density (at 1 at. %) 1.38 × 10²⁰ cm⁻³ 1.38 × 10²⁰ 0.93 × 10²⁰ Upperstate lifetime 230 μs 520 μs 360 μs Lower state lifetime <200 ps  10 nsThermal cond. (W/cm- .14 .06 .006 K) Laser wavelength (nm) 1064.1 10531047 1053.5 Index of refraction 1.82 1.4481 1.4704 1.508 Emission crosssection 2.8 × 10⁻¹⁹ cm² 1.2 × 10⁻¹⁹ 1.8 × 10⁻¹⁹ 4.2 × 10⁻²⁰ cm² Absorpcross section 2.9 × 10^(−20 cm) ² 2.2 × 10⁻²⁰ 6.5 × 10⁻²⁰ 2.4 × 10⁻²⁰Absorption peak 808 nm 797 nm 792 nm 808 nm Absorp. coef (1 at. %) 4cm⁻¹ 3 cm⁻¹ 9 cm⁻¹ 2.2 cm⁻¹

Several embodiments of the current invention are specifically designedto address the need for efficient, high average power laser operation atapertures greater than approximately one square centimeter. Efficientoperation is achieved in continuous wave lasers by operating the lasernear the saturation intensity, I_(sat)=hν/σ_(e)τ. For the case of thematerials described in TABLE 3, this ranges from 2.9 (Nd:YAG) to over 12kW/cm² (Nd:Glass). Thus, according to one embodiment of the invention, alaser device is provided which has an aperture greater than 1 cm² and anaverage power of at least 1 kW/cm², although it is understood that laserdevices may be made in accordance with the invention that haveconsiderably less output power. No conventional solid-state laser couldoperate at this level of average pump power and size. Laser operation insuch conventional solid state lasers would cease rapidly as thetemperature increased in the absence of cooling. The laser materialwould have to be cooled by a fluid flowing over its surface. All knownsolid-state laser materials would fracture due to thermal stress orseverely distort the beam resulting from the large temperature gradientwhich would be established between the center region of the laser andthe cooled surface at an aperture greater than approximately 1 cm andpumped by this level of average diode power. The present inventionsolves this problem by having one dimension of the solid-state laserelements sufficiently small that the temperature gradient establishedbetween the center of the element and the surface is small. Furthermore,the temperature of the gain medium elements is readily controlled bysimply controlling the temperature of the fluid by conventional heatexchanger technology which is well known in the art. These features arequantified by calculating the temperature distribution inside a thinsheet element. In the limit that the thickness of the element 14 is muchsmaller than either its width or height, we can approximate thetemperature distribution across the element as being described byone-dimensional heat flow,

${\frac{\partial^{2}T}{\partial x^{2}} + \frac{q^{\prime\prime\prime}}{k_{s}}} = 0$where k_(s) is the thermal conductivity of the solid laser medium, x isthe distance measured from the center of the element 14 toward itssurface in the thin dimension of the element (see FIG. 6), and q′″ isthe local heat generation within the volume. Heat is conductedthroughout the element 14 and is transferred to the fluid by convectionat the surface. This is described by the surface boundary condition,

${{- k_{s}}\frac{\partial T}{\partial x}} = {h\left( {T_{s} - T_{f}} \right)}$where h is the surface heat transfer coefficient, T_(s) represents thetemperature of the element 14 at its surface and T_(f) represents thetemperature of the fluid 16 far away from the element surface. Theseequations can be readily solved to show that the steady-statetemperature distribution across the thin dimension of the element 14 isparabolic with a maximum temperature difference between the center ofthe element 14 and its surface ΔT of,

${\Delta\; T} = \frac{q^{\prime\prime\prime}L^{2}}{{ck}_{s}}$where c is a geometric constant depending on the geometry of the element14. For example, c can be a number roughly between 4 and 8, e.g., c≅8for a sheet or plate element. Thus, in a sheet element, ΔT is providedby:

${\Delta\; T} = \frac{q^{\prime\prime\prime}L^{2}}{8k_{s}}$

In the limit of low fractional excitation of the active laser ions inthe solid-state medium (i.e., non-saturated excitation), volumetric heatgeneration, q′″(W/cm³) occurs throughout the element 14 at a rate givenby,q′″=I_(pump)α_(a)δwhere I_(pump) is the irradiance of the diode pump radiation, α_(a) isthe absorption coefficient described earlier and δ is known as thequantum defect. The quantum defect is the difference in energy betweenthe absorption and emission bands of the laser medium. For Neodymium,photons absorbed near 803 nm have an energy of 1.54 eV and the emissionnear 1060 nm have an energy of 1.17 eV, quantum defect, δ≈(1.54 eV-1.17eV)/1.54 eV =24%. In other words for every photon absorbed from thepump, at least 0.37 eV is left in the solid-state material as heat. Fora pump flux of I_(pump)=400 W/cm² and an absorption coefficient of 9cm⁻¹, we will have heat generated inside the element 14 at a rate ofq′″=864 W/cm³. When the fraction of ions excited is large, a moreappropriate estimate of the volumetric heat generation rate is,q′″=N _(ex) δE _(pump)/τwhere N_(ex) is the upper state excited ion density, E_(pump) is theenergy of absorbed pump photons and τ is the lifetime of the upper laserlevel in the solid-state material.

According to several embodiments of the invention, the elements 14 aredesigned to have at least one dimension that is significantly less thanthe other dimensions to allow heat removal from the element 14 throughconduction to the surface and then convection to a flowing fluid 16. Forexample, sheet shaped elements should be designed sufficiently thin toallow the generated heat to be removed at a rate such that the maximumtemperature reached within the solid and the gradient remainsufficiently low to enable efficient lasing and good beam quality. It isnoted that the dimension designed to be sufficiently small may be athickness of a sheet or a diameter of a cylinder, for example. Solvingthe above equations for L, which is the small dimension of the element(e.g., thickness illustrated in FIG. 6), the maximum thickness of theelement 14 in order to ensure adequate heat removal is given by:

$L \leq \sqrt{\frac{\Delta\;{Tck}_{s}}{q^{''\prime}}}$This equation enables one of ordinary skill in the art to design a laseroscillator or amplifier based on the present invention simply by knowingthe basic thermo-optical properties of the solid-state laser material(e.g., thermal conductivity, k_(s), absorption cross section, σ_(a), iondensity, N, and the quantum defect, δ) and the irradiance of the diodearrays I_(pump). As described previously, it is the pump irradiance, theabsorption of the material, and the quantum defect which determine thevolumetric heat generation rate, q′″ in the material. The maximumallowable ΔT will vary for different laser materials but is typicallyless than 100° C.

In general terms in order to achieve high power laser operation, e.g.,power above 100 W, the difference in temperature between a center of anelement 14 and its surface, (ΔT in the equations above) should be lessthan approximately 100° C. for most solid-state laser materials. Thus,solving the equation for ΔT above provides a maximum quantity to thesmall dimension of the element 14 such that the element 14 will besufficiently thin for heat removal. For example, in the case of asheet-like element 14, for a maximum temperature difference ΔT of 100°C., the thickness of the element 14 in order to ensure adequate heatremoval is given by:

$L \leq \sqrt{\frac{800k_{s}}{q^{''\prime}}}$

The significance of the several embodiments of the present inventionlies in the dependency of the temperature on the square of the dimensionof the solid-state laser media, e.g., elements 14. As a specificexample, consider the use of Nd:YLF as the solid-state laser material.The thermal conductivity of Nd:YLF is k_(s)=6.0 W/m ° C. From the aboveequations, we see that the temperature difference between the center ofthe element and its surface, ΔT=T(0)−T_(s)=q′″L²/8k_(s) is less than 20°C. for a one millimeter thick element 14 pumped at 400 W/cm³ and anabsorption coefficient of 9 cm⁻¹. Note that a conventional rod with adiameter of the order of 1 cm pumped under the same conditions wouldexhibit a characteristic thermal difference between the center and theedge of several thousand degrees with catastrophic failure occurringmuch earlier. Furthermore, since heat is conducted out of the elementspredominantly along the direction of the laser axis, the effect of thetemperature gradient on the wavefront quality of the laser beam is muchless than in conventional rod or slab based systems in which the heat isconducted transverse to the laser axis.

As such, according to several embodiments, a laser device is providedthat can be configured to operate at high average power and exhibits amaximum temperature much less than is achievable in current state of theart diode-pumped, solid-state lasers. This is due in part to therelative sizing of the solid state elements 14 as distributed throughoutthe laser chamber 12 and cooling fluid 16 flowing about the elements 14such that the laser emission passes through the elements 14 and thefluid 16. In some embodiments, the fluid is substantially index matchedto the elements 14 at the relevant laser emission wavelength and therelevant optical pump source wavelength; thus, the majority of the pumpradiation is absorbed by the solid state elements 14. Since the laserdevice is generally not limited by temperature rise in the solid statematerial, the laser device may be operated in a pulsed manner or in amore continuous manner. For example, a laser device in accordance withseveral embodiments of the invention may be operated continuously forvery long periods of time, such as, greater than 1 second to fullycontinuous. That is, the optical pump radiation from the semiconductordiode array is continuously pumped into the gain medium for a durationof at least the time desired for laser output. This output time islimited only by the ability to supply pump power to the gain medium andremove heat from the fluid in the heat exchanger.

Referring next to FIG. 7, an example how an embodiment of a laser devicein accordance with the invention can be used in a four-pass amplifier.Here, the beam emerges from an oscillator 102 producing a pulsed output.The beam passes through an expanding telescope 104 to adjust the beamsize to approximate that of the aperture of the gain medium. The pulsethen passes through a thin film polarizer 106 oriented to passP-polarization. The pulse then passes through a Faraday Rotator 108which rotates the polarization by 45 degrees. A half-waveplate or 45degree quartz rotator 110 cancels the rotation of the Faraday rotator108 orienting the beam back at P-polarization for passage through thesecond thin-film polarizer 112. The beam then passes through aquarter-waveplate 114 oriented to provide circular polarization at theoutput. The beam is then directed to (e.g., via mirrors 116 and 118) andthen passes through the laser chamber 12 of a laser device 10 accordingto an embodiment of the present invention whereby it undergoesamplification. After the first-pass of amplification, the beam strikes amirror 120 which directs it back through the gain medium (e.g., solidstate elements 14 described herein) for a second pass. The second passthrough the quarter-waveplate 114 converts the circularly polarized beamto linearly polarized in the S plane (vertical). The now S-polarizedbeam reflects off the thin film polarizer 112 towards another mirror122. This mirror reflects the beam back through the waveplate andamplifier combination. The beam strikes the first mirror 120 a secondtime and passes a fourth time through the amplifiers. Passage throughthe quarter-waveplate 114 this time produces a linearly-polarized beamin the P (horizontal) plane which passes through thin-film polarizer112. The beam then encounters the half-waveplate 110 which rotates theplane of polarization by 45 degrees. Passage through the Faraday rotatorin the backwards direction rotates the polarization by an additional 45degrees in the direction produced by the waveplate 110. The Faradayrotator/waveplate combination serves to rotate the plane of polarizationby 90 degrees when the beam is travelling backwards toward theoscillator. The now S-polarized beam then reflects off of thin-filmpolarizer 106 and is directed out of the laser system. This is one ofmany possible uses of an embodiment of the present invention in amultipass amplifier. Similar uses of other embodiments of the inventionas a single-pass amplifier, inside a regenerative amplifier design or asthe gain medium in an oscillator cavity are not intended to be precludedby this example.

Changes and modifications in the specifically described embodiments canbe implemented without departing from the scope of the invention, whichis intended to be limited only by the scope of the appended claims.

1. A laser device comprising: a laser chamber having a volume formedtherein; a gain medium within the volume and comprising a solid-stateelement containing active laser ion within the volume, the solid-stateelement rigidly held in a fixed position within the volume; a fluidflowing about the solid-state element; a semiconductor laser diode forproviding optical pump radiation into the volume of the laser chamber;and a laser emission from the solid state element passes through thegain medium and the fluid; wherein a dimension L of the solid stateelement is defined by:$L \leq \sqrt{\frac{\Delta\;{Tck}_{S}}{q^{\prime''}}}$ where ΔT is amaximum temperature difference between a center and a surface of thesolid state element, k_(s) is a thermal conductivity of the solid stateelement, c is a geometric constant wherein c is a value between 4 and 8,and q′″ is a heat generated per unit volume within the solid stateelement resulting from diode-pumping.
 2. The laser device of claim 1wherein the solid state element comprises a sheet, wherein c is equal to8.
 3. The laser device of claim 1 wherein the dimension of the solidstate element is sufficiently thin such that in operation at a powerlevel of at least 100 W, a temperature difference between the center andthe surface of the solid state element across the dimension is at most100 degrees C.
 4. The laser device of claim 1 wherein a dimension thesolid state element is between about 10 microns and 2 mm.
 5. The laserdevice of claim 1 wherein the gain medium produces the laser emission atan average power of at least 100 W.
 6. The law device of claim 1 whereinlaser chamber further comprises an inlet and an outlet, the fluid flowedthrough the laser chamber from the inlet to the outlet.
 7. The laserdevice of claim 6 further comprising a heat exchanger coupled to theinlet and outlet, the fluid exiting the outlet passing through the heatexchanger and cooled then flowed through the inlet.
 8. The laser deviceof claim 1 wherein the fluid is flowed through the laser chamber in adirection transverse to the direction of laser emission.
 9. The laserdevice of claim 1 wherein the solid-state element comprise one of aplurality of flat sheets distributed throughout the volume, the fluidflowing in between each of the flat sheets.
 10. The laser device ofclaim 9 wherein each of the flat sheets is oriented at an angle withrespect to the direction of a laser emission and at an angle withrespect to the optical pump radiation.
 11. The laser device of claim 10wherein a spacing between each of the flat sheets is such that each ofthe flat sheets receives substantially the same amount of optical pumpradiation from the semiconductor laser diode.
 12. The laser device ofclaim 10 wherein a spacing between each of the flat sheets is such thata portion of each flat sheet overlaps a portion of an adjacent flatsheet relative to the direction of the optical pump radiation.
 13. Thelaser device of claim 10 wherein a spacing between each of the flatsheets is such that adjacent flat sheets do not overlap each otherrelative to the direction of the optical pump radiation.
 14. The laserdevice of claim 1 wherein a refractive index of the fluid at awavelength of the laser emission substantially matches the refractiveindex of the solid state element.
 15. The laser device of claim 1wherein the fluid comprises a carbon-based material.
 16. The laserdevice of claim 1 wherein the semiconductor laser diode providescontinuous optical pump radiation in order to provide a continuous laseremission for greater than 1 second.
 17. The laser device of claim 1wherein the semiconductor laser diode provides pulsed optical pumpradiation in order to provide a pulsed laser emission.
 18. The laserdevice of claim 1 wherein the solid-state element is oriented at or nearBrewster's angle with respect to a path of the laser emission.
 19. Thelaser device of claim 1 further comprising a manifold adapted to receivethe fluid to be flowed through the laser chamber and including aplurality of openings coupled to a plurality of inlets to the laserchamber distributed across a dimension of the laser chamber, themanifold providing substantially uniform flow of the fluid about thesolid-state element.
 20. A method of lasing comprising: providing alaser chamber having a volume formed therein and containing a gainmedium, the gain medium comprising a solid state element containingactive laser ion within the volume, the solid state element rigidly heldin a fixed position within the volume; flowing a cooling fluid throughthe volume and about the solid state element; directing optical pumpradiation produced by semiconductor laser diodes through the chamberinto the volume; and directing a laser emission produced by the solidstate element through the chamber such that the laser emission passesthrough the solid state element and the fluid; wherein the providingstep comprises providing the laser chamber containing the gain medium,wherein a dimension L of the solid state element is defined by:$L \leq \sqrt{\frac{\Delta\;{Tck}_{S}}{q^{\prime''}}}$ where ΔT is amaximum temperature difference between a center and a surface of thesolid state element k_(s) is a thermal conductivity of the solid stateelement, c is a geometric constant wherein c is a value between 4 and 8,and q′″ is a heat generated per unit volume within the solid stateelement.
 21. The method of claim 20 further comprising: cooling aportion of the cooling fluid flowing out of the chamber and flowing theportion back into the laser chamber.
 22. The method of claim 20 furthercomprising: providing the optical pump radiation sufficient to operatethe laser device at a power level of at least 100 W, wherein atemperature difference between the center and the surface of the solidstate element across the dimension across the solid state element is atmost 100 degrees C.
 23. The method of claim 20 wherein the directingstep comprises: directing pulsed optical pump radiation produced bysemiconductor laser diodes through the chamber into the volume.
 24. Themethod of claim 20 wherein the directing step comprises: directingcontinuous optical pump radiation produced by semiconductor laser diodesthrough the chamber into the volume.
 25. A laser device comprising:means for flowing a cooling fluid through a volume formed in a laserchamber and about a solid state element, the laser chamber containing again medium, the gain medium comprising the solid state elementcontaining active laser ion within the volume, the solid state elementrigidly held in a fixed position within the volume, wherein a dimensionL of the solid state element is defined by:$L \leq \sqrt{\frac{\Delta\;{Tck}_{S}}{q^{\prime''}}}$ where ΔT is amaximum temperature difference between a center and a surface of thesolid state element k_(s) is a thermal conductivity of the solid stateelement, c is a geometric constant wherein c is a value between 4 and 8,and q′″ is a heat generated per unit volume within the solid stateelement; means for directing optical pump radiation produced bysemiconductor laser diodes through the chamber into the volume; andmeans for directing a laser emission produced by the solid state elementthrough the chamber such that the laser emission passes through thesolid state element and the fluid.